Wireless local area networks (WLANs) have been developed for various commercial and residential applications. Such networks allow for mobile terminals to be moved within a particular service area without regard to a physical connection to the network. To enable communications in the WLAN, various wireless standards have been developed, including the IEEE 802.11x standard, Blue Tooth, and HIPERLAN, to name a few. A WLAN typically includes an Access Point (AP) and one or more stations. Each station may be a device such as a notebook computer, personal digital assistant (PDA), wireless VoIP telephone, or the like that transmits radio signals to and receives radio signals from other stations in the local area network via the AP.
In an IEEE 802.11(e) wireless communication standard, a mode of operation for enhanced quality of service (QoS) has been provided. The mode of operation, called a hybrid coordination function, allows a QoS control entity to have prioritized access to the communication medium. The QoS control entity, called hybrid controller (HC), can be located in an IEEE 802.11 AP. The prioritized access to the medium allows the HC to allocate transmission opportunities to voice stations via a polling mechanism, known as contention-free polling. Therefore, in an IEEE 802.11(e) WLAN, voice packet polled transmission opportunities can be periodically granted to an 802.11 communication device.
Unlike callers in a circuit-switched telephone network who are allocated a dedicated connection, users of wireless standards, such as IEEE 802.11(e), do not occupy a fixed amount of bandwidth during their online sessions but instead share the network with other online users, only using network resources when sending and receiving data in quick bursts. Users typically grab what they need from the available bandwidth at the instant they need to upload their packets. A module that generates packets, such as a digital signal processor (DSP) is typically separate from the module that transmits the data. Thus, transmission of packets can typically include packet wait times as well as latency, especially when the different modules reside on different physical hardware and are not running synchronized clocks. Delays in the transmission of voice packets can significantly degrade voice quality.
FIG. 1 demonstrates a prior art example of a timing diagram 10 associated with the transmission of packets in a wireless communication device. The timing diagram 10 can represent, for example, transmission of packets in a wireless communication device in an IEEE 802.11(e) WLAN. An HC in an IEEE 802.11(e) WLAN provides polled transmission opportunities, demonstrated in the example of FIG. 1 as TXMIT, at predetermined time intervals N1. The production and transmission of packets, such as voice packets, from a digital signal processor (DSP) to a transmission device can thus be performed at the predetermined time intervals N2, such that the packets are produced and transmitted to the transmission device at fixed time slots. The predetermined time intervals N1 and N2 can be substantially equal, such that each polled transmission opportunity can be used to transmit a packet.
At a time T0, a packet is generated and transmitted from the DSP to the transmission device, demonstrated in the example of FIG. 1 as TPKT. At a time T1, the packet is received by the transmission device. A propagation delay (i.e., latency) associated with the time that it takes for the packet to be transmitted from the DSP to the transmission device is demonstrated by the time interval P. However, the next polled transmission opportunity for the transmission device to transmit the packet does not occur until the time T2. At the time T2, the transmission device transmits the packet, demonstrated in the example of FIG. 1 as TXMIT. The transmission of the packet from the transmission device is delayed by a wait time interval W. Thus, a delay time interval D includes both the propagation delay interval P and the wait time interval W.
At the time T3, the next packet is generated and transmitted from the DSP to the transmission device. Accordingly, the next packet arrives at the transmission device at the time T4, and is transmitted from the transmission device at the time T5. The generation and transmission of the next packet is again repeated at the times T6, T7, and T8.
In this prior art example of FIG. 1, the production and transmission of the packets, as well as the polled transmission opportunities, are separated by the substantially equal time intervals N1 and N2. As such, the delay time interval D is constant. As demonstrated in the example of FIG. 1, the wait time interval W is the same between the times T1 and T2, the times T4 and T5, and the times T6 and T8. As such, each of the transmitted packets has an additional delay of W. For voice packets, this additional delay can cause a significant reduction of voice quality.